Method for making light emitting diode

ABSTRACT

A method for making light emitting diode includes following steps. A substrate having an epitaxial growth surface is provided. A first semiconductor layer, an active layer, and a second semiconductor layer is epitaxially grown on the epitaxial growth surface of the substrate in that sequence. A cermet layer is formed on the second semiconductor layer. The substrate is removed to form an exposed surface. A first electrode is applied to cover the entire exposed surface of the first semiconductor layer. A second electrode is applied to electrically connected to the second semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201210185696.0, filed on Jun. 7, 2012, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. This application is related tocommonly-assigned applications entitled, “SEMICONDUCTOR STRUCTURE”,filed on Dec. 28, 2012, U.S. Pat. No. 13/729,588 “LIGHT EMITTING DIODE”,filed on Dec. 28, 2012, U.S. Pat. No. 13/729,617 the contents of theabove commonly-assigned applications are hereby incorporated byreference.

BACKGROUND

1. Technical Field

The present disclosure relates to semiconductor structures, a lightemitting diode, and a method for making the same.

2. Description of Related Art

Semiconductor structures fabricated by gallium nitride for lightsources, such as blue, green, and white light sources, have longlifetime, high energy conversion efficiency, and are green. Therefore,the semiconductor structures are widely used as the light sources inlarge screen color display systems, automotive lighting, traffic lights,multimedia displays, optical communication systems, and so on.

A semiconductor structure used in a standard light emitting diode (LED)includes an N-type semiconductor layer, a P-type semiconductor layer,and an active layer located between the N-type semiconductor layer andthe P-type semiconductor layer. In operation, a positive voltage and anegative voltage are applied respectively to the P-type semiconductorlayer and the N-type semiconductor layer. Thus, holes in the P-typesemiconductor layer and electrons in the N-type semiconductor layer canenter the active layer and combine with each other to emit visiblelight, and the visible light is emitted from the semiconductorstructure. However, near field evanescent waves emitted from the activelayer are internally reflected inside the semiconductor structure, sothat a large portion of the light emitted from the active layer remainin the semiconductor structure, thereby degrading the light extractionefficiency of the LED.

What is needed, therefore, is to provide a semiconductor structure forsolving the problem discussed above.

BRIEF DESCRIPTION OF THE DRAWING

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, the emphasis instead being placed upon clearlyillustrating the principles of the present embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a schematic sectional view of one embodiment of asemiconductor structure.

FIG. 2 is a schematic sectional view of one embodiment of asemiconductor structure.

FIG. 3 is a schematic sectional view of one embodiment of asemiconductor structure.

FIG. 4 is a schematic sectional view of one embodiment of asemiconductor structure.

FIG. 5 is a schematic sectional view of one embodiment of a threedimensional structure array of FIG. 4.

FIG. 6 is a scanning electron microscope image of FIG. 5.

FIG. 7 is a schematic view of one embodiment of an LED.

FIG. 8 is a flowchart of one embodiment of a method for making the LEDof FIG. 7.

FIG. 9 is a schematic view of one embodiment of an LED.

FIG. 10 is a flowchart of one embodiment of a method for making the LEDof FIG. 9.

FIG. 11 is a schematic view of one embodiment of an LED.

FIG. 12 is a schematic view of one embodiment of an LED.

FIG. 13 is a schematic view of one embodiment of a solar cell.

FIG. 14 is a schematic view of one embodiment of a wave guide tube.

FIG. 15 is a figure that shows a relationship between the Angularfrequency and the Purcell Factor of the LED of FIG. 7.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone.

Referring to FIG. 1, one embodiment of a semiconductor structure 10includes a substrate 110 having an epitaxial growth surface 112, abuffer layer 116, a first semiconductor layer 120, an active layer 130,a second semiconductor layer 140, a protective layer 150, and a cermetlayer 160. The buffer layer 116, the first semiconductor layer 120, theactive layer 130, the second semiconductor layer 140, the protectivelayer 150, and the cermet layer 160 are stacked on the epitaxial growthsurface 112 in that sequence.

The substrate 110 can be a transparent structure having an epitaxialgrowth surface 112 used to grow the first semiconductor layer 120. Theepitaxial growth surface 112 is a smooth surface. Oxygen and carbon areremoved from the epitaxial growth surface 112. The substrate 110 can bea single layer structure or a multiple layer structure. If the substrate110 is a single layer structure, the substrate 110 can be asingle-crystal structure. The single-crystal structure includes acrystal face which is used as the epitaxial growth surface 112. Amaterial of the substrate 110 can be silicon on insulator (SOI), LiGaO₂,LiAlO₂, Al₂O₃, Si, GaAs, GaN, GaSb, InN, InP, InAs, InSb, AlP, AlAs,AlSb, AlN, GaP, SiC, SiGe, GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN, AlInN,GaAsP, InGaN, AlGaInN, AlGaInP, GaP:Zn, or GaP:N. If the substrate 110is the multiple layer structure, the substrate 110 should include atleast one layer of the single-crystal structure mentioned previously.The material of the substrate 110 can be selected according to the firstsemiconductor layer 120. In one embodiment, a lattice constant andthermal expansion coefficient of the substrate 110 is similar to thefirst semiconductor layer 120 thereof to improve a quality of the firstsemiconductor layer 120. In one embodiment, the material of thesubstrate 110 is sapphire. A thickness, shape, and size of the substrate110 are arbitrary and can be selected according to need.

The buffer layer 116 is sandwiched between the substrate 110 and thefirst semiconductor layer 120. A thickness of the buffer layer 116ranges from about 10 nanometers to about 300 nanometers, such as about20 nanometers and about 50 nanometers. A material of the buffer layer116 can be GaN, or AlN with low melting point. In one embodiment, thematerial of the buffer layer 116 is GaN, and the thickness of the bufferlayer 116 is about 20 nanometers.

A thickness of the first semiconductor layer 120 can be in a range fromabout 1 micrometer to about 15 micrometers. The first semiconductorlayer 120 can be a doped semiconductor layer. The doped semiconductorlayer can be an N-type semiconductor layer or a P-type semiconductorlayer. A material of the N-type semiconductor layer can be at least oneof N-type GaN, N-type GaAs, and N-type cupric phosphide. A material ofthe P-type semiconductor layer can be at least one of P-type GaN, P-typeGaAs, and P-type cupric phosphide. The N-type semiconductor layer isconfigured to provide electrons, and the P-type semiconductor layer isconfigured to provide holes. In one embodiment, the material of thefirst semiconductor layer 120 is the N-type GaN doped with Si element,and the thickness of the first semiconductor layer 120 is about 1460nanometers.

The active layer 130 is a photon excitation layer to provide a locationfor the combination of the electrons and holes. Photons are produced inthe active layer 130 when the electrons and holes are combined. Theactive layer 130 can be one of a single layer quantum well film ormultilayer quantum well film. A material of the quantum well film can beat least one of GaInN, AlGaInN, GaAs, GaAlAs, GaInP, InAsP, and InGaAs.A thickness of the active layer 130 can be in a range from about 0.01micrometers to about 0.6 micrometers. In one embodiment, the material ofthe active layer 130 is a composition of InGaN and GaN, and thethickness of the active layer 130 is about 10 nanometers.

The second semiconductor layer 140 can be the N-type semiconductor layeror the P-type semiconductor layer. The type of the first semiconductorlayer 120 and the type of the second semiconductor layer 140 isdifferent to form a PN conjunction. The second semiconductor layer 140is located on the active layer 130. A thickness of the secondsemiconductor layer 140 ranges from about 5 nanometers to about 250nanometers. In one embodiment, the thickness of the second semiconductorlayer 140 ranges from about 10 nanometers to about 200 nanometers. Inone embodiment, the thickness of the second semiconductor layer 140 isin a range from about 10 nanometers to about 30 nanometers. Thethickness of the second semiconductor layer 140 can be about 10nanometers, about 50 nanometers, about 100 nanometers, or about 200nanometers. In one embodiment, the second semiconductor layer 140 is theP-type GaN doped with Mg element, and the thickness of the secondsemiconductor layer 140 is about 10 nanometers. In one embodiment, therefractive index of the composite semiconductor layer 200 is about 2.5.

The protective layer 150 is located on the second semiconductor layer140. The protective layer 150 is sandwiched between the secondsemiconductor layer 140 and the cermet layer 160. The protective layer150 can prevent the metallic plasma generated by the cermet layer 160from converting to heat. The refractive index of the cermet layer 160under a guided wave mode is a complex number including a real part andan imaginary part. If the imaginary part is large, the metallic plasmais easily converted to heat. Thus, the protective layer 150 is made of amaterial with a low refractive index and can reduce the real part andthe imaginary part at the same time. Thus, the heat consumption of themetallic plasma can be reduced, and the metallic plasma can travelfarther. Therefore, the extraction efficiency of the semiconductorstructure 10 can be increased. In one embodiment, the refractive indexof the protective layer 150 can be in a range from about 1.2 to about1.5. In one embodiment, the refractive index of the protective layer 150is in a range from about 1.3 to about 1.4. In one embodiment, therefractive index of the protective layer 150 is in a range from about1.4 to about 1.5. The material of the protective layer 150 can besilicon dioxide, magnesium fluoride, or lithium fluoride. In oneembodiment, the material of the protective layer 150 is silicon dioxide,and the refractive index of the protective layer 150 is about 1.5.

A thickness of the protective layer 150 can be in a range from about 5nanometers to about 40 nanometers. In one embodiment, the thickness ofthe protective layer 150 is in a range from about 5 nanometers to about10 nanometers. In one embodiment, the thickness of the protective layer150 is in a range from about 10 nanometers to about 20 nanometers. Inone embodiment, the thickness of the protective layer 150 is in a rangefrom about 20 nanometers to about 30 nanometers. In one embodiment, thethickness of the protective layer 150 is in a range from about 30nanometers to about 40 nanometers. The thinner the protective layer 150,the closer the cermet layer 160 and active layer 130. Therefore, theinteraction between the cermet layer 160 and the active layer 130 willbe improved, and more photos will be generated by the active layer 130.In one embodiment, the thickness of the protective layer 150 is about 20nanometers.

A material of the cermet layer 160 can be selected to ensure that thecermet layer 160 can generate metallic plasma. Therefore, the materialof the cermet layer 160 is metallic in optics. Furthermore, the materialof the cermet layer 160 can satisfy the following requirements. First,the refractive index of the cermet layer 160 is a complex numberincluding a real part and an imaginary part, and the imaginary partcannot be zero. Second, a dielectric constant of the cermet layer 160 isa complex number including a real part and an imaginary part, and thereal part is a negative number.

The material of the cermet layer 160 can be a metallic composite, andthe metallic composite can include a metal material and a dielectricmaterial, such as metal ceramic. Thus lights with long wavelengths canbe easily extracted from the active layer 130. The metal can be silver,aluminum, copper or gold. The alloy can be gold-silver alloy,gold-aluminum alloy, or silver-aluminum alloy. The dielectric materialcan be silicon, silicon dioxide, or ceramic. A mass percent of thedielectric material in the metallic composite can be less than or equalto 40%, such as about 35%, about 30%, about 25%, about 20%, about 15%,about 10%, or about 5%. In one embodiment, the metal in the cermet layer160 is silver, and the dielectric material is silicon dioxide. Athickness of the cermet layer 160 can be in a range from about 50nanometers to about 100 nanometers. The thickness of the cermet layer160 can be selected to ensure that the photons generated by the activelayer 130 can be extracted out through the substrate 110. In oneembodiment, the thickness of the cermet layer 160 is about 50nanometers.

Furthermore, the substrate 110 is optional and can also be omitted fromthe semiconductor structure 10.

The cermet layer 160 has many functions. Near field evanescent wavesgenerated by the active layer 130 can be amplified and converted tometallic plasma by the cermet layer 160, when arriving at the cermetlayer 160. The metallic plasma can be extracted out of the semiconductorstructure 10 through the substrate 110. A few metallic plasma can bescattered by the cermet layer 160 and spread around the cermet layer160. While the metallic plasma reaches the active layer 130, themetallic plasma can interact with the quantum well in the active layer130 to activate more secondary photons. The secondary photons can alsoarrive at the cermet layer 160 and introduce more metallic plasma. Thus,more photons can be extracted out of the semiconductor structure 10under the interaction between the cermet layer 160 and the active layer130. Furthermore, the uniformity of the current distributed in thesecond semiconductor layer 140 can be improved.

Also referring to FIG. 2, one embodiment of a semiconductor structure 20is provided. The semiconductor structure 20 includes a substrate 110having an epitaxial growth surface 112, a buffer layer 116, a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, and a cermet layer 160. The buffer layer 116, the firstsemiconductor layer 120, the active layer 130, the second semiconductorlayer 140, and the cermet layer 160 are stacked on the epitaxial growthsurface 112 in that sequence.

The semiconductor structure 20 is similar to the semiconductor structure10, except that the protective layer 150 is omitted. The cermet layer160 is directly located on a surface of the second semiconductor layer140. Thus a distance between the cermet layer 160 and the active layer130 is reduced, and the interaction between them is improved.

Referring to FIG. 3, one embodiment of a semiconductor structure 30includes a substrate 110 having an epitaxial growth surface 112, a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, a protective layer 150, and a cermet layer 160. The firstsemiconductor layer 120, the active layer 130, the second semiconductorlayer 140, the protective layer 150, and the cermet layer 160 arestacked on the epitaxial growth surface 112 in that sequence.

The semiconductor structure 30 is similar to the semiconductor structure10, except that the buffer layer 116 is omitted. The first semiconductorlayer 120 is directly located on the epitaxial growth surface 112 of thesubstrate 110.

Referring to FIG. 4, one embodiment of a semiconductor structure 40 isprovided. The semiconductor structure 40 includes a first semiconductorlayer 120, an active layer 130, a second semiconductor layer 140, aprotective layer 150, and a cermet layer 160 stacked in that order. Thesemiconductor structure 40 is similar to the semiconductor structure 20,except that a surface of the second semiconductor layer 140 away fromthe active layer 130 includes a plurality of primary three-dimensionalnano-structures 143.

Also referring to FIG. 5, the second semiconductor layer 140 can beseparated into a main body 140 a and a protruding part 140 b anddistinguished by an “interface.” The interface can be parallel with thefirst surface of the second semiconductor layer 140. The interface isconfigured as a surface of the main body 140 a, and the protruding part140 b is extending away from the interface. The protruding part 140 bdefines the plurality of primary three-dimensional nano-structures 143,and the plurality of primary three-dimensional nano-structures 143 formthe patterned surface of the second semiconductor layer 140. Thethree-dimensional nano-structure 143 can be a protruding structure. Theprotruding structure protrudes out from the interface of the main body140 a. The plurality of primary three-dimensional nano-structures 143 isa protruding structure located on the interface of the main body 140 a.

The plurality of primary three-dimensional nano-structures 143 can bearranged side by side. Each of the primary three-dimensionalnano-structures 143 can extend along a straight line, a curvy line, or apolygonal line. The extending direction is substantially parallel withthe surface of the second semiconductor layer 140. The two adjacentthree-dimensional nano-structures are arranged a certain distance apartfrom each other. The distance ranges from about 0 nanometers to about1000 nanometers, such as about 10 nanometers, about 30 nanometers, orabout 200 nanometers. The extending direction of the three-dimensionalnano-structure 143 can be fixed or varied. While the extending directionof the three-dimensional nano-structure 143 is fixed, the plurality ofprimary three-dimensional nano-structures 143 extends along a straightline. Otherwise the primary three-dimensional nano-structures 143extends along a polygonal line or a curvy line. The cross-section of thethree-dimensional nano-structure 143 along the extending direction isM-shaped. Referring to FIG. 5 and FIG. 6, the primary three-dimensionalnano-structures 143 are a plurality of substantially parallel bar-shapedprotruding structures extending along a straight line. The plurality ofprimary three-dimensional nano-structures 143 are substantiallyuniformly and equidistantly distributed on the entire surface of themain body 140 a.

The three-dimensional nano-structure 143 extends from one side of thesecond semiconductor layer 140 to the opposite side along the Xdirection. The Y direction is substantially perpendicular to the Xdirection and substantially parallel with the surface of the main body140 a. The three-dimensional nano-structure 143 is a double-peakstructure including two peaks. The cross-section of the double-peakstructure is in the shape of an M. Each M-shaped three-dimensionalnano-structure 143 includes a first peak 1132 and a second peak 1134.The first peak 1132 and the second peak 1134 extend substantially alongthe X direction. A first groove 1136 is defined between the first peak1132 and the second peak 1134. A second groove 1138 is defined betweentwo adjacent primary three-dimensional nano-structures 143.

The first peak 1132 and the second peak 1134 protrude out of the mainbody 140 a. The height of the first peak 1132 and the second peak 1134is arbitrary and can be selected according to need. In one embodiment,both the height of the first peak 1132 and that of the second peak 1134range from about 150 nanometers to about 200 nanometers. The height ofthe first peak 1132 can be substantially equal to that of the secondpeak 1134. The highest points of the first peak 1132 and the second peak1134 are defined as the farthest point away from the surface of the mainbody 140 a. In one three-dimensional nano-structure 143, the highestpoint of the first peak 1132 is spaced from that of the second peak 1134a certain distance ranging from about 20 nanometers to about 100nanometers. The first peak 1132 and the second peak 1134 extendsubstantially along the X direction. The cross-section of the first peak1132 and the second peak 1134 can be trapezoidal or triangular, and theshape of the first peak 1132 and the second peak 1134 can besubstantially the same. In one embodiment, the cross-sections of thefirst peak 1132 and the second peak 1134 are triangular. In oneembodiment, the first peak 1132, the second peak 1134, and the main body140 a form an integrated structure.

In each M-shaped three-dimensional nano-structure 143, the first peak1132 and the second peak 1134 define the first groove 1136. Theextending direction of the first groove 1136 is substantially the sameas the extending direction of the first peak 1132 and the second peak1134. The cross-section of the first groove 1136 is V-shaped. The depthh₁ of the first groove 1136 in each three-dimensional nano-structure 143is substantially the same. The depth h₁ is defined as the distancebetween the highest point of the first peak 1132 and the lowest point ofthe first groove 1136. The depth of the first groove 1136 is less thanthe height of the first peak 1132 and the second peak 1134.

The second groove 1138 extends substantially along the extendingdirection of the primary three-dimensional nano-structures 143. Thecross-section of the second groove 1138 is V-shaped or an inversetrapezium. Along the extending direction, the cross-section of thesecond groove 1138 is substantially the same. The depth h₂ of the secondgrooves 1138 between adjacent primary three-dimensional nano-structures143 is substantially the same. The depth h₂ is defined as the distancebetween the highest point and the lowest point of the groove of thesecond groove 1138. The depth of the second groove 1138 is greater thanthe depth of the first groove 1136, and the ratio between h₁ and h₂ranges from about 1:1.2 to about 1:3 (1:1.2≦h₁:h₂≦1:3). The depth of thefirst groove 1136 ranges from about 30 nanometers to about 120nanometers, and the depth of the second groove 1138 ranges from about 90nanometers to about 200 nanometers. In one embodiment, the depth of thefirst groove 1136 is about 80 nanometers, and the depth of the secondgroove 1138 is about 180 nanometers. The depth of the first groove 1136and the second groove 1138 can be selected according to need.

The protective layer 150 and the cermet layer 160 are stacked on thepatterned surface of the second semiconductor conductive layer 140.Furthermore, the surface of the protective layer 150, and the surface ofthe cermet layer 160 away from the active layer 130 can also form asecondary patterned surface. The secondary patterned surface is similarto the patterned surface of the second semiconductor layer 140. Thesecondary patterned surface also includes a plurality of secondarythree-dimensional nano-structures 163, and the distribution and thealignment of the secondary three-dimensional nano-structures 163 <Fixedin SPEC and FIG, is that ok?> are the same as the distribution and thealignment of primary three-dimensional nano-structures 143.

Furthermore, if the protective layer 150 is omitted, the cermet layer160 is directly located on the surface of the second semiconductor layer140. The surface of the cermet layer 160 away from the cermet layer 160can also form the plurality of secondary three-dimensionalnano-structures 163.

The semiconductor structure has many advantages. First, the plurality ofprimary three-dimensional nano-structures 143 change the motiondirection of the photons reaching the light emitting surface with alarge incident angle, so that these photons can be extracted from thelight emitting surface. Second, the metallic plasma can be scattered bythe plurality of well aligned three-dimensional nanostructures 143 onthe cermet layer, thus the metallic plasma can be easily extracted.Third, because the three-dimensional nano-structure 143 is M-shaped, theM-shaped primary three-dimensional nano-structures 143 can function astwo layers of the three-dimensional nano-structure assembled together.The light extraction efficiency of the semiconductor structure will beimproved.

Furthermore, the surface of the first semiconductor layer 120 adjacentto the active layer 130 can also be a patterned surface including aplurality of three-dimensional nanostructures (now shown). Therefore,the contact area between the first semiconductor layer 120 and theactive layer 130 can be enlarged. The electron-hole recombinationdensity is improved, and the quantity of photons is increased. The lightextraction efficiency of the semiconductor structure 30 can be improved.

Also referring to FIG. 7, one embodiment of an LED 50 includes a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, and a cermet layer 160 stacked in that order. The firstsemiconductor layer 120 includes a first surface 121 and a secondsurface 122 opposite to the first surface. The active layer 130 islocated on the second surface 122. A first electrode 124 covers anddirectly contacts the first surface 121 of the first semiconductor layer120 away from the active layer 130. A second electrode 144 iselectrically connected to the second semiconductor layer 140. In oneembodiment, a part of the surface of the second semiconductor layer 140is exposed, and the second electrode 144 is located on the exposedsurface of the second semiconductor layer 140.

The first electrode 124 is electrically connected with the firstsemiconductor layer 120. The first electrode 124 can cover the entiresurface of the first semiconductor layer 120 away from the active layer130. Thus the LED 50 forms a vertical structure, and the currentdiffusion speed will be improved and the heat produced in the LED 50will be decreased. The first electrode 124 is a single layer structureor a multi-layer structure. The first electrode 124 can be an N-typeelectrode or a P-type electrode according to the first semiconductorlayer 120. The material of the first electrode 124 can be selected fromTi, Ag, Al, Ni, Au, or any combination of them. The material of thefirst electrode 124 can also be indium-tin oxide (ITO) or carbonnanotube film. In one embodiment, the first electrode 124 is a two-layerstructure consisting of a Ti layer with about 15 nm in thickness and anAu layer with about 100 nm in thickness.

The second electrode 144 can be an N-type electrode or a P-typeelectrode. The type of the second electrode 144 is the same as thesecond semiconductor layer 140. The shape of the second electrode 144 isarbitrary and can be selected according to need. The second electrode144 covers at least part of the surface of the second semiconductorlayer 140. The second electrode 130 can cover at least part of thethree-dimensional nano-structures 142. The shape and the location of thesecond electrode 144 cannot affect the light extraction efficiency ofthe LED 50. The second electrode 144 is a single layer structure or amulti-layer structure. The material of the second electrode 144 can beselected from Ti, Ag, Al, Ni, Au or any combination of them. Thematerial of the second electrode 144 can also be ITO. In one embodiment,the second electrode 144 is transparent to reduce the reflectivity andthe absorption, thus improving the light extraction efficiency.

Also referring to FIG. 8, one embodiment of a method for making the LED50 includes the following steps:

(S610), providing a substrate 110 having an epitaxial growth surface112;

(S620), growing a first semiconductor layer 120, an active layer 130,and a second semiconductor layer 140 on the epitaxial growth surface 112in that order;

(S630), applying a cermet layer 160 on the second semiconductor layer140;

(S640), removing the substrate 110 to expose a surface of the firstsemiconductor layer 120; and

(S650), applying a first electrode 124 to cover the exposed surface ofthe first semiconductor layer 120, and a second electrode 144electrically connected to the second semiconductor layer 140.

In step (S620), the first semiconductor layer 120, the active layer 130and the second semiconductor layer 140 can be grown respectively via aprocess of molecular beam epitaxy (MBE), chemical beam epitaxy (CBE),vacuum epitaxy, low temperature epitaxy, choose epitaxy, liquid phasedeposition epitaxy (LPE), metal organic vapor phase epitaxy (MOVPE),ultra-high vacuum chemical vapor deposition (UHVCVD), hydride vaporphase epitaxy (HYPE), and metal organic chemical vapor deposition(MOCVD).

In one embodiment, the first semiconductor layer 120 is Si-doped N-typeGaN. The first semiconductor layer 120 is made by a MOCVD method, andthe growth of the first semiconductor layer 120 is a heteroepitaxialgrowth. In the MOCVD method, the nitrogen source gas is high-purityammonia (NH₃), the carrier gas is hydrogen (H₂), the Ga source gas istrimethyl gallium (TMGa) or triethyl gallium (TEGa), and the Si sourcegas is silane (SiH₄). The growth method of the active layer 130 issimilar to the first semiconductor layer 120. In one embodiment, theindium source gas is trimethyl indium. The second semiconductor layer140 is grown after the growth of the active layer 130. In oneembodiment, the Mg source gas is ferrocene magnesium (Cp₂Mg).

In step (S630), the cermet layer 160 can be deposited on the secondsemiconductor layer 140 by sputtering or vacuum evaporation. In oneembodiment, the material of the cermet layer 160 includes silver andsilicon dioxide. The cermet layer 160 can be deposited on the secondsemiconductor layer by the following method:

(S631), providing a melted silver and a melted silicon dioxide;

(S632), forming a melted alloy by mixing the melted silver and themelted silicon dioxide, wherein the mass percent of the melted silicondioxide is about 20%; and

(S633), evaporating the melted alloy and depositing the melted alloy onthe second semiconductor layer 140.

In step (S640), the substrate 110 can be removed by laser irradiation,etching, or thermal expansion and contraction. The removal method can beselected according to the material of the substrate 110 and the firstsemiconductor layer 120. In one embodiment, the substrate 110 is removedby laser irradiation. The substrate 110 can be removed from the firstsemiconductor layer 120 by the following steps:

(S641) polishing and cleaning the surface of the substrate 110 away fromthe first semiconductor layer 120;

(S642) placing the substrate 110 on a platform (not shown) andirradiating the substrate 110 and the first semiconductor layer 120 witha laser; and

(S643) immersing the substrate 110 into a solvent and removing thesubstrate 110.

In step (S641), the substrate 110 can be polished by a mechanicalpolishing method or a chemical polishing method to obtain a smoothsurface. Thus the scatting of the laser will be decreased. The substrate110 can be cleaned with hydrochloric acid or sulfuric acid to remove themetallic impurities and oil.

In step (S642), the substrate 110 is irradiated by the laser from thepolished surface, and the incidence angle of the laser is substantiallyperpendicular to the surface of the substrate 110. The wavelength of thelaser is selected according to the material of the first semiconductorlayer 120 and the substrate 110. The energy of the laser is smaller thanthe bandgap energy of the substrate 110 and larger than the bandgapenergy of the first semiconductor layer 120. Thus the laser can passthrough the substrate 110 and reach the interface between the substrate110 and the first semiconductor layer 120. The buffer layer at theinterface has a strong absorption of the laser, and the temperature ofthe buffer layer will be raised rapidly. Thus the buffer layer will bedecomposed. In one embodiment, the bandgap energy of the firstsemiconductor layer 120 is about 3.3 eV, and the bandgap energy of thesubstrate 110 is about 9.9 eV. The laser is a KrF laser, the wavelengthof the laser is about 248 nm, the energy is about 5 eV, the pulse widthrange is about 20 nanoseconds to about 40 nanoseconds, the energydensity ranges from about 400 mJ/cm² to about 600 mJ/cm², and the shapeof the laser pattern is square with a size of about 0.5 mm×0.5 mm. Thelaser moves from one edge of the substrate 110 with a speed of about 0.5mm/s. During the irradiating process, the GaN is decomposed to Ga andN₂. The parameter of the laser and the wavelength of the laser can beselected according to the absorption of the buffer layer.

Because the buffer layer has a strong absorption of the laser, thebuffer layer can decompose rapidly. However, the first semiconductorlayer 120 has a weak absorption, so it does not decompose quickly. Theirradiating process can be performed in a vacuum or a protective gasenvironment. The protective gas can be nitrogen, helium, argon, or otherinert gas.

In step (S643), the substrate 110 can be immersed into an acidicsolution to remove the Ga decomposed from GaN so that the substrate 110can be peeled off from the first semiconductor layer 120. The acidicsolution can be hydrochloric acid, sulfuric acid, or nitric acid whichcan dissolve the Ga.

In step (S650), the first electrode 124 and the second electrode 144 canbe formed via a process of physical vapor deposition, such as electronbeam evaporation, vacuum evaporation, ion sputtering, or any physicaldeposition. Furthermore, a conductive substrate (not shown) can bedirectly attached on and electrically connected to the firstsemiconductor layer 120, and the conductive substrate can be used as thefirst electrode 124.

Also referring to FIG. 9, one embodiment of an LED 60 includes a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, a protective layer 150, and a cermet layer 160 stacked inthat order. The first semiconductor layer 120 includes a first surface121 and a second surface 122 opposite to the first surface. The activelayer 130 is located on the second surface 122. A first electrode 124covers and directly contacts the first surface 121 of the firstsemiconductor layer 120 away from the active layer 130. A secondelectrode 144 is electrically connected to the second semiconductorlayer 140. In one embodiment, a part surface of the second semiconductorlayer 140 is exposed, and the second electrode 144 is located on theexposed surface of the second semiconductor layer 140.

The structure of the LED 60 is similar to the structure of the LED 50,except that the LED 60 further includes a protective layer 150sandwiched between the second semiconductor layer 140 and the cermetlayer 160.

Also referring to FIG. 10, one embodiment of a method for making the LED60 includes the following steps:

(S810), providing a substrate 110 having an epitaxial growth surface112;

(S820), growing a first semiconductor layer 120, an active layer 130,and a second semiconductor layer 140 on the epitaxial growth surface 112in that order;

(S830), applying a protective layer 150 on the second semiconductorlayer 140;

(S840), applying a cermet layer 160 on the protective layer 150;

(S850), removing the substrate 110 to expose a surface of the firstsemiconductor layer 120; and

(S860), applying a first electrode 124 to cover the exposed surface ofthe first semiconductor layer 120, and a second electrode 144electrically connected to the second semiconductor layer 140.

The method of making the LED 60 is similar to the method of making theLED 50, except an additional step of locating the protective layer 150on the second semiconductor layer 140. The protective layer 150 can bedeposited on the second semiconductor layer 140 via sputtering, chemicalvapor deposition, or vacuum evaporation. In one embodiment, a materialof the protective layer 150 is silicon dioxide, and the thickness of theprotective layer 150 is about 130 nanometers. The protective layer 150can be deposited on the second semiconductor layer 140 via chemicalvapor deposition.

Also referring to FIG. 11, one embodiment of an LED 70 includes a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, a protective layer 150, and a cermet layer 160 stacked inthat order. The first semiconductor layer 120 includes a first surface121 and a second surface 122 opposite to the first surface. The activelayer 130 is located on the second surface 122. A first electrode 124covers and directly contacts the first surface 121 of the firstsemiconductor layer 120 away from the active layer 130. A secondelectrode 144 is electrically connected to the second semiconductorlayer 140.

The structure of the LED 70 is similar to the structure of the LED 60,except that the second semiconductor layer 140 further includes aplurality of primary three-dimensional nano-structures 143. Theplurality of primary three-dimensional nano-structures 143 is applied onthe surface of the second semiconductor layer 140 away from the activelayer 130. The structure of the plurality of primary three-dimensionalnano-structures 143 of LED 70 is similar to that in the semiconductorstructure 30. Thus the light extractive efficiency of LED 70 will beimproved. Referring to FIG. 11, the light extractive efficiency of LED70 is much greater than that of the standard LED and the standard onlywith three-dimensional nano-structures.

Furthermore, the plurality of three-dimensional nano-structures can alsobe applied on a surface of the first semiconductor layer 120 adjacent tothe active layer 130, or a surface of the active layer 130 adjacent tothe second semiconductor layer 140. Therefore, the contact area betweenthe first semiconductor layer 120 and the active layer 130, or betweenthe second semiconductor layer 140 and the active layer 130, can beenlarged. The electron-hole recombination density is improved, and thequantity of photons is increased. The light extraction efficiency of theLED 70 can be improved.

The method of one embodiment of making the LED 70 includes:

(S710), providing a substrate 110 having an epitaxial growth surface112;

(S720), growing a first semiconductor layer 120, an active layer 130,and a second semiconductor layer 140 on the epitaxial growth surface 112in that order;

(S730), forming a plurality of three-dimensional nanostructures 133 on asurface of the second semiconductor layer 140 away from the active layer130;

(S740), forming a protective layer 150 on the plurality of primarythree-dimensional nano-structures 143;

(S750), applying a cermet layer 160 on the protective layer 150;

(S760), removing the substrate 110 to expose a surface of the firstsemiconductor layer 120; and

(S770), applying a first electrode 124 to cover the exposed surface ofthe first semiconductor layer 120, and a second electrode 144electrically connected to the second semiconductor layer 140.

The method of making LED 70 is similar to the method of making LED 60,except the method further comprises a step of forming a plurality ofprimary three-dimensional nano-structures 143 on the secondsemiconductor layer 140.

In step (S730), the plurality of primary three-dimensionalnano-structures 143 can be formed by:

-   -   (S731), locating a mask layer on the surface of the second        semiconductor layer 140;    -   (S732), patterning the mask layer by a nanoimprinting and        etching method;    -   (S733), patterning the surface of the second semiconductor layer        140 by an etching method to form a plurality of        three-dimensional nano-structure performs; and    -   (S734), forming the plurality of the primary three-dimensional        nano-structures 143 by removing the mask layer.

In step (S731), the mask layer can be a single layered structure or amulti-layered structure. In one embodiment, the mask layer is themulti-layered structure including a first mask layer and a second masklayer disposed on a surface of the first mask layer. The first masklayer and the second mask layer are stacked on the surface of the secondsemiconductor layer 140 in sequence. A material of the first mask layeris ZEP520A® which is developed by Zeon Corp of Japan, a material of thesecond mask layer is HSQ (hydrogen silsesquioxane).

In step (S732), the mask layer can be patterned by the following steps:

-   -   (S7321), providing a patterned template, which includes a        plurality of protruding structures spaced from and substantially        parallel with each other, and a slot is defined between the two        adjacent protruding structure;    -   (S7322), attaching the template on the second mask layer, and        pressing the template at room temperature and removing the        template to form a plurality of slots on the second mask layer;    -   (S7323), removing the residual second mask layer in the bottom        of the slot to expose the first mask layer, and    -   (S7324), patterning the mask layer by removing one part of the        first mask layer corresponding with the slots to expose the        second semiconductor layer 140.

In step (S733), the second semiconductor layer 140 can be placed in aninductively coupled plasma device and etched by an etching gas. In oneembodiment, the etching gas is a mixed gas. The mixed gas can includeCl₂, BCl₃, O₂ and Ar. A power of the inductively coupled plasma deviceranges from about 10 watts to about 100 watts, a flow speed of theetching gas ranges from about 8 sccm to about 150 sccm, a pressure ofthe etching gas can range from about 0.5 Pa to about 15 Pa, and anetching time can range from about 5 seconds to about 5 minutes. In oneembodiment, the flow speed of the Cl₂ is about 26 sccm, the flow speedof the BCl₃ is about 16 sccm, the flow speed of the O₂ is about 20 sccm,and the flow speed of the Ar is about 10 sccm.

More specifically, the second semiconductor layer 140 can be etched bythe following steps:

-   -   (S7331), forming a plurality of grooves with the same depth by        etching the surface of second semiconductor layer 140 with the        etching gas;    -   (S7332), continuing the etching process so that every two        adjacent protruding structures begin to slant face to face to        form a protruding pair; and    -   (S7333), further continuing the etching process so that the two        adjacent protruding structures gradually slant until the tops of        the two adjacent protruding structures contact each other.

In step (S7331), the etching gas etches the exposed surface of thesecond semiconductor layer 140 to form the plurality of grooves. Thegrooves have the same depth because of the same etching speed. Duringthe etching process, the etching gas will react with the exposed secondsemiconductor layer 140 to form a protective layer. The protective layerwill reduce the etching speed of the second semiconductor layer 140, andthe width of the grooves will slowly decrease from the outer surface ofthe second semiconductor layer 140 to the bottom of the grooves. Thus,the inner wall of the grooves will not be absolutely perpendicular tothe surface of the second semiconductor layer 140. The etching gas notonly etches the second semiconductor layer 140, but also etches the topof the protruding structures. The width of the top of the protrudingstructures will decrease. The resolution of the mask layer will not beaffected because the speed of etching the top of the protrudingstructures is much smaller than that of the second semiconductor layer140. Furthermore, every two adjacent protruding structures will slantface to face.

In step (S7332), the tops of the two adjacent protruding structures willgradually approach to each other. The speed of etching the secondsemiconductor layer 140 corresponding to these two closed adjacentprotruding structures will decrease, and the width of the grooves willgradually decrease from the outer surface of the second semiconductorlayer 140 to the bottom of the grooves of the second semiconductor layer140. Because the two adjacent protruding structures slant face to faceto form the protruding pair, the speed of etching the secondsemiconductor layer 140 corresponding to the protruding pair willfurther decrease. Eventually, the tops of the two adjacent protrudingstructures contact each other, and the etching gas can no longer etchthe second semiconductor layer 140 corresponding to the two adjacentprotruding structures, thus the first grooves 1136 is formed on thesurface of the second semiconductor layer 140. But between every twoadjacent protruding pairs, the etching speed will change less than theetching speed between the slant two adjacent protruding structures. Thusthe second grooves 1138 are formed, and the depth of the second grooves1138 will be greater than that of the first grooves 1136. The pluralityof three-dimensional nano-structure preforms is obtained.

In step (S734), the primary three-dimensional nano-structures 143 can beobtained by dissolving the mask layer. The mask layer can be dissolvedin a stripping agent such as tetrahydrofuran (THF), acetone, butanone,cyclohexane, hexane, methanol, or ethanol. In one embodiment, thestripping agent is butanone, and the mask layer is dissolved in butanoneand separated from the second semiconductor layer 140.

The plurality of primary three-dimensional nano-structures 143 also canbe formed on the surface of the active layer 130 away from the firstsemiconductor layer 120 or the surface of the first semiconductor layer120 adjacent to the active layer 130 by the above method. The protectivelayer 150 and the cermet layer 160 are directly grown on the surface ofthe plurality of three-dimensional nanostructures 133, thus theprotective layer 150 and the cermet layer 160 also form a patternedsurface.

In the method for making the LED 70, the nanoimprinting and etchingmethod is used to form the plurality of primary three-dimensionalnano-structures 143. The nanoimprinting process can be conducted in aroom temperature and the template can be directly used without beingpre-treated. Therefore, the method has a simple process and low cost. Inaddition, a large area array of the plurality of M-shaped primarythree-dimensional nano-structures 143 can be fabricated for the twoadjacent protruding structures of the mask layer can be contact witheach other by the gas etching to form the plurality of protruding pairs.Therefore, a yield of the LED 70 can be increased.

Referring to FIG. 12, one embodiment of an LED 80 includes a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, a protective layer 150, and a cermet layer 160 stacked inthat order. The first semiconductor layer 120 includes a first surface121 and a second surface 122 opposite to the first surface. The activelayer 130 is located on the second surface 122. A reflector layer 170covers and directly contacts a surface of the cermet layer 160 away fromthe active layer 130. A first electrode 124 covers and directly contactsthe first surface 121 of the first semiconductor layer 120 away from theactive layer 130. A second electrode 144 is electrically connected tothe second semiconductor layer 140.

The structure of LED 80 is similar to the structure of the LED 70,except that the structure of the LED 80 further includes the reflectorlayer 170 located on the cermet layer 160. The material of the reflectorcan be selected from Ti, Ag, Al, Ni, Au, or any combination thereof. Thereflector layer 170 includes a smooth surface having a highreflectivity. The photons reach the reflector layer 170 and will bereflected, thus these photons can be extracted out of the LED 70, andthe light extraction efficiency of the LED 70 through the firstsemiconductor layer 120 can be improved. The reflector layer 170 canalso be a plurality of microstructures on the surface of the cermetlayer 160. The microstructure can be a groove and a bulge. Furthermore,the first electrode 124 can be transparent.

Referring to FIG. 13, one embodiment of a solar cell 22 usingsemiconductor structure 10 includes a first collecting electrode 32, thesubstrate 110, the buffer layer 116, the first silicon layer 126, aphotovoltaic layer 136, a second silicon layer 146, a protective layer150, a cermet layer 160, and a second collecting electrode 34. Thesubstrate 110, the buffer layer 116, the first silicon layer 126, thephotovoltaic layer 136, the second silicon layer 146, the protectivelayer 150, and the cermet layer 160 are stacked on a surface of thefirst collecting electrode 32 in the listed sequence.

The semiconductor type of the first silicon layer 126 and the secondsilicon layer 146 are different. The first silicon layer 126 can beN-typed or P-typed. In one embodiment, the first silicon layer 126 isP-typed, and the second silicon layer 146 is N-typed.

The first collecting electrode 32 and the second collecting electrode 34can be made with the same material or different materials. The firstcollecting electrode 32 or the second collecting electrode 34 can be ametal plate with a continuous surface. The material of the firstcollecting electrode 32 or the second collecting electrode 34 can beselected from the group consisting of Al, Cu, Ag, and combinationsthereof. A thickness of the first collecting electrode 32 or the secondcollecting electrode 34 can be in a range from about 50 nanometers toabout 300 nanometers. In one embodiment, both the first collectingelectrode 32 and the second electrode 34 are bar-shaped Al foils withthe thickness of about 200 nanometers.

The solar cell 22 includes a light-input surface 310 and a light-outputsurface 320. One side of the substrate 110, the buffer layer 116, thefirst silicon layer 126, the photovoltaic layer 136, the second siliconlayer 146, the protective layer 150, and the cermet layer 160 constitutethe light-input surface 310. An opposite side of the side of thesubstrate 110, the buffer layer 116, the first silicon layer 126, thephotovoltaic layer 136, the second silicon layer 146, the protectivelayer 150, and the cermet layer 160 constitute the light-output surface320 of the solar cell 22.

Sunlight irradiates the light-input surface 310 and reaches the cermetlayer 160. The metallic plasma is then generated from the cermet layer160. The metallic plasma is absorbed by a P-N conjunction formed by thefirst silicon layer 126 and the second silicon layer 136 to form largeramounts of electrons and holes. The electrons move to the secondcollecting electrode 34 and the holes move to the first collectingelectrode 32 to form a current.

Referring to FIG. 14, one embodiment of a waveguide tube 44 is provided.The waveguide tube 44 is substantially the same as the semiconductorstructure 10. The cermet layer 160 of the waveguide tube 44 includes twosubstantially parallel and opposite sides: a first side 161 and a secondside 162. Electromagnetic waves can enter from the first side 161 andpass through from the second side 162. The metallic plasma is generatedwhen the electromagnetic waves reaches the cermet layer 160. Themetallic plasma carrying information of the electromagnetic wavesspreads in the cermet layer 160 and then converted back to theelectromagnetic waves when reaching the second side 162. Therefore, thewaveguide tube 44 can conduct the electromagnetic waves.

Referring to FIG. 15, a curve between the Angular frequency and thePurcell Factor of the LED 50 of FIG. 7 is shown. A mass percent of thesilicon dioxide in the cermet layer 160 is defined as factor “A”. Thefactor A can be 0%, 10%, 20%, 30%, 40%. A dotted line shows aphotoluminescence intensity of LED 50. The smaller the distance betweenthe curve line and the dotted line is, the more metallic plasma can beactivated in the cermet layer 160. Thus, while the factor A is about 20%or about 30%, the cermet layer 160 generates more metallic plasma, andthe light extraction efficiency of the LED 50 can be improved.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the present disclosure. Variations maybe made to the embodiments without departing from the spirit of thepresent disclosure as claimed. Elements associated with any of the aboveembodiments are envisioned to be associated with any other embodiments.The above-described embodiments illustrate the scope of the presentdisclosure but do not restrict the scope of the present disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A method for making a light emitting diode, themethod comprising: providing a substrate having an epitaxial growthsurface; epitaxially growing a first semiconductor layer, an activelayer, and a second semiconductor layer on the epitaxial growth surfaceof the substrate in that sequence; forming a cermet layer on the secondsemiconductor layer, wherein the cermet layer is in direct physicalcontact with the second semiconductor layer; exposing a surface of thefirst semiconductor layer by removing the substrate to form an exposedsurface; applying a first electrode covering the entire exposed surfaceof the first semiconductor layer; and applying a second electrodeelectrically connected to the second semiconductor layer.
 2. Thesemiconductor structure of claim 1, wherein a material of the cermetlayer comprises a metal material and a dielectric material.
 3. Themethod of claim 2, wherein a mass percent of the dielectric material issmaller than or equal to 40%.
 4. The method of claim 2, wherein themetal material is selected from the group consisting of gold, silver,aluminum, copper, and alloy thereof.
 5. The method of claim 2, whereinthe dielectric material is selected form the group consisting ofsilicon, silicon dioxide, and ceramic.
 6. The method of claim 1, whereinthe cermet layer comprises silver and silicon dioxide.
 7. The method ofclaim 1, wherein the cermet layer is deposited on the secondsemiconductor layer via sputtering or vacuum evaporation in the step offorming the cermet layer.
 8. The method of claim 1, wherein a thicknessof the cermet layer ranges from about 10 nanometers to about 30nanometers in the step of forming the cermet layer.
 9. The method ofclaim 1, further comprising a step of forming a plurality of primarythree-dimensional nano-structures on the second semiconductor layer. 10.The method of claim 9, wherein the plurality of primarythree-dimensional nanostructures is formed on a surface of the secondsemiconductor layer away from the active layer.
 11. The method of claim10, wherein the plurality of primary three-dimensional nano-structuresis formed by: locating a patterned mask layer on a surface of the secondsemiconductor layer, wherein the patterned mask layer comprises aplurality of bar-shaped protruding structures aligned side by side, anda slot defined between each two adjacent protruding structures of theplurality of bar-shaped protruding structures to expose a portion of thesecond semiconductor layer; etching the exposed portion of the firstsemiconductor layer, wherein the each two adjacent protruding structuresbegin to slant face to face until they are contacting each other to forma protruding pair; and removing the mask layer.
 12. The method of claim11, wherein a first part of the second semiconductor layer correspondingto each of the protruding pairs is etched at a first speed, and a secondpart of the second semiconductor layer exposed from the each twoadjacent protruding pairs is etched at a second speed, and the firstspeed is less than the second speed.
 13. The method of claim 12, whereina first groove is defined in the first part of the first semiconductorlayer, and a second groove is defined in the second part of the firstsemiconductor layer.
 14. The method of claim 10, wherein the cermetlayer is deposited on the plurality of primary three-dimensionalnano-structures.
 15. The method of claim 14, wherein a plurality ofsecondary three-dimensional nano-structures is formed on a surface ofthe cermet layer away from the second semiconductor layer.
 16. Themethod of claim 15, wherein the plurality of primary three-dimensionalnano-structures and the plurality of secondary three-dimensionalnano-structures have the same alignments and extending directions.
 17. Amethod for making a light emitting diode, the method comprising:providing a substrate having an epitaxial growth surface; epitaxiallygrowing a first semiconductor layer, an active layer, and a secondsemiconductor layer on the epitaxial growth surface of the substrate inthat sequence; forming a protective layer on the second semiconductorlayer, wherein the protective layer is in contact with the secondsemiconductor layer; forming a cermet layer on the protective layer,wherein the cermet layer covers entire surface of the protective layeraway from the second semiconductor layer; exposing a surface of thefirst semiconductor layer by removing the substrate to form an exposedsurface; applying a first electrode covering the entire exposed surfaceof the first semiconductor layer; and applying a second electrodeelectrically connected to the second semiconductor layer.
 18. The methodof claim 17, further comprising a step of forming a plurality of primarythree-dimensional nano-structures on the second semiconductor layerbefore the step of forming the protective layer.
 19. The method of claim18, wherein a plurality of secondary three-dimensional nano-structuresis formed on the protective layer in the step of forming the protectivelayer.
 20. The method of claim 19, wherein the plurality of secondarythree-dimensional nano-structures is formed on the cermet layer in thestep of forming the cermet layer.